1. Field of the Invention
This invention relates to lasing methods and apparatus, and more particularly to Q-switched lasers in which energy is absorbed in the system to produce a high energy pulse.
2. Description of the Related Art
Single laser pulses of high peak power have been achieved by introducing into the laser cavity an irradiance-dependent or time-varying loss. If there is initially a high loss in the laser cavity, the gain from the inverted population can reach a very high value without laser oscillation beginning. Laser oscillation is prevented by the loss, while energy is pumped to the excited state of the active lasing medium. When a large population inversion is reached, the cavity losses are deliberately and suddenly reduced. The threshold gain decreases immediately, but the actual gain remains high because of the large excited-state population. The large difference between actual gain and threshold causes the laser radiation in the cavity to quickly grow, and all available energy is emitted in a single, large pulse. This pulse rapidly depopulates the excited state to such an extent that the gain is reduced below its threshold value, and lasing action ceases. This sudden altering of the losses of a laser cavity is known as Q-switching, because of the close relationship between resonator losses and resonator Q.
Passive Q-switching using saturable absorption in diode lasers was demonstrated by Lee and Roldan, "Repetitively Q-Switched Light Pulses From GaAs Injection Lasers with Random Double-Section Striped Geometry", IEEE Journal of Quantum Electronics, June, 1970, pages 339-352. Active Q-switching using electro-absorption by a buried heterostructure was described in an article by Tsang, et al., "Q-Switching of Low-Threshold BuriedHeterostructure Diode Lasers at 10 GHz", Applied Physics Letters, Vol. 45, No. 3, Aug. 1, 1984, pages 204-206. In both of these approaches the Q-switching acts as an energy drain to prevent lasing, and significantly lowers the efficiency of the laser.
Another active Q-switched laser is described in Arakawa, et al., "Active Q-switching In a GaAs/AlGaAs Multiquantum Well Laser with an Intracavity Monolithic Loss Modulator", Applied Physics Letters, Vol. 48, No. 9, Mar. 3, 1986, pages 561-563. A multiquantum well (MQW) laser was formed, consisting of an optical amplifier section and an electro-absorption loss modulator section. MQWs are periodic structures consisting of alternating ultrathin layers constructed of two semiconductors with different electrical and optical properties, but crystal structures with nearly identical lattice spacings; the matching lattices insure a continuous crystal with few defects. MQWs may be formed by molecular beam epitaxy, typically with layer thicknesses on the order of 100 Angstroms or more. A common semiconductor pair for this purpose is gallium arsenide and aluminum gallium arsenide. MQWs are described, for example, in Robinson, "Multiple Quantum Wells for Optical Logic", Science, Vol. 225, Aug. 24, 1984, pages 822-825.
In the Arakawa, et al. device, output pulses are periodically obtained by applying an electric field across the MQW structure. The parameters are selected such that a loss modulation is achieved through the quantum confined Stark effect in the modulator section, which together with a band-gap shrinkage that occurs in the amplifier section is said to result in extremely large loss changes. While pulses as narrow as 18.6 ps full width at half-maximum are said to be generated with a high repetition rate of more than 3 Ghz, this system is similar to the Lee and Tsang approaches in that the amplification of electro-absorption to MQW lasers results in a Q-switch that acts only as an energy drain, thereby significantly limiting the overall efficiency.
The operation of an MQW under the influence of an applied electric field E varies considerably, depending upon the particular MQW structure. MQWs have been categorized according to the relationship between the valance and conduction bands of their constituent semiconductors. The energy gap between the top of the valance band and the bottom of the conduction band is referred to as the band-gap. Type I MQWs are those in which the valance and conduction bands of the narrower gap material are nested within the band-gap of the other material; in other words, the narrower band-gap material has a lower conduction band but higher valance band than the other material. This combination is referred to as "nested alignment". Type I MQWs include heterostructures of the form GaAs/Al.sub.x Aa.sub.1-x As, in which x is less than 0.40. The MQW employed in the Arakawa, et al. Q-switched laser falls into this category.
In Type II MQWs, either the valance or conduction band of the narrower band-gap material, or both the valance and conduction bands, lie outside the band-gap of the other semiconductor. In staggered alignment Type II systems, either the valance or conduction band of the narrower gap material, but not both, lies outside the other material's band-gap; in misaligned Type II systems, both the valance and conduction bands of the narrower band-gap material lie outside the band-gap of the other material.
FIG. 1 is a simplified illustration of a MQW in cross-section, in which two semiconductors 2 and 4 with the required crystal near-match are alternated. The response of the MQW to an electric field E perpendicular to the layers is illustrated in FIGS. 2a and 2b, and is described in an article by Wilson, "Carrier Dynamics and Recombination Mechanisms in Staggered-Alignment Heterostructures," IEEE Journal of Ouantum Electronics, Vol. 24, No. 8, August, 1988, pages 1763-1777. The band-gap structure is illustrated in FIG. 2a, in which C1, V1, C2 and V2 represent the conduction and valance bands of the first and second semiconductor materials, respectively. When the material is excited, as by the application of electromagnetic radiation, electrons are photoexcited from the higher valance band material (VI) into the conduction band for that material (C1), leaving a hole in V1. The charge carriers are trapped in the smaller band-gap material, and recombination occurs by the excited electron dropping in energy directly from C1 back to V1.
A staggered alignment Type II system is illustrated in FIG. 2b. In this system the valance band V1' of the larger band-gap material is at an energy level less than the valance band V2' of the narrower band-gap material, while the conduction band C1' for the wider band-gap material is also at an energy level less than the conduction band C2' of the narrower band-gap material. Thus, the lowest energy excited states for photoexcited electron and holes occur in opposite layers of the structure. As a result, excited electrons move laterally as a result of electron-lattice scattering from C2' to the lower conduction band c1', and thus become spatially separated from their recombination partners as indicated in FIG. 2b. The holes do not have an effective lateral movement because their effective mass is much greater than that of an electron. Subsequent recombination must take place across the interface between the two materials, which substantially reduces the recombination rate.
The application of an electric field perpendicular to the layers of a staggered alignment Type II MQW is illustrated in FIG. 3. The band structure becomes skewed, with the narrower band-gap material undergoing a red shift and the wider band-gap material a blue shift. With a sufficient applied field, the opposite motion of the energy shifts for the two conduction bands will cause the bands to align, or even reverse their relative positions. This induces a sudden retransfer of excited electron population back to the narrower band material, which in turn produces a strong population inversion and an optical emission burst as the excited electrons rapidly recombine with the holes in the valance band of the same material. As opposed to a recombination rate before the field is applied on the order of 1 microsecond - 1 msec, the electron retransfer rate with the field applied is much less than 1 psec.
As described in the literature, sharp pulses can be produced from staggered alignment Type II MQWs by first irradiating the structure so that it stores energy in the form of photoexcited electrons spatially separated from their recombination sites, and then applying an electric field to release the stored energy in a sudden burst, the wavelength of which is determined by the band-gap across which recombination occurs. This output is observed as a non-directional pulse extending essentially over 360.degree., as in Meynadier, et al., "IndirectDirect Anticrossing in GaAs-AlAs Superlattices Induced by an Electric Field: Evidence of .GAMMA.-x Mixing", The American Physical Society, Vol. 60, No. 13, Mar. 28, 1988 (pages 1338-1341). The device in effect functions as an omni-directional light source, without the collimation coherence offered by a laser.